My girlfriend is very cool with all of my geeky car stuff. I'm a structural engineer, so she's used to me trying to explain ridiculous things to her, and wanting perfection out of all of my toys, from my car to my gaming PC to snowboarding gear, etc.
I think she's intimidated by the thought of modding things, but she's said she wishes she had a car that she could build up like I'm doing with mine. I usually can't shut my mouth though whenever I get into a new mod, so as cool as she is, I still try to make her feel like she's dating me... not me and the car.
And at the end of the day, a nice little pull in 2nd or 3rd and she's all giggles.